The typical prior art exhaust muffler comprises separate tubes supported in a parallel array by a plurality of transversely extending baffles. Selected tubes in the array are provided with perforations, louvers, apertures or the like to permit a controlled expansion of exhaust gases travelling through the tubes. The assembly of tubes and baffles are slid within a generally tubular outer shell having an oval or circular cross section corresponding to the shape of the baffles. An outer wrapper may be wrapped around and secured to the tubular outer shell of the muffler to dampen vibrations of the tubular outer shell and prevent vibration related noise. End caps are securely affixed to opposed ends of the tubular outer shell and outer wrapper to substantially enclose the muffler. Each end cap typically will be provided with one or more apertures to define at least one inlet and at least one outlet for the muffler. This prior art construction provides a plurality of chambers within the muffler. In particular, the chambers are defined between the tubular outer shell and either two adjacent baffles or between one baffle and an end cap.
The dimensions and relative spacing of the various components within the above described prior art muffler are selected in accordance with engine specifications and operating performance. For example, the diameters of the respective tubes in the muffler may be selected in accordance with the flow rates of exhaust gases at different engine operating conditions, and in accordance with a specified allowable back pressure that may be created by the muffler.
Certain chambers within these prior art mufflers will define expansion chambers having perforated of louvered tubes extending therethrough. The volume encompassed by the expansion chamber and the total area encompassed by the perforations or louvers in the tubes will be selected in accordance with the noise characteristics exhibited by the flow of exhaust gas from the engine. Generally the expansion chambers will be constructed to attenuate a very broad range of the noise. However, one or more fairly narrow bands of low frequency noise typically will remain despite the broad attenuation achieved by the expansion chamber. Thus, the typical prior art muffler will comprise at least one low frequency resonating chamber into which a tuning tube extends. The volume of the low frequency resonating chamber and the length and cross sectional area of the tuning tube will be selected in accordance with the particular low frequency sound to be attenuated. Many mufflers will require two physically and functionally separate low frequency resonating chambers and tuning tube combinations to attenuate two distinct ranges of low frequency sounds.
The dimensions and spacial disposition of the components in the prior art muffler may be determined by an acoustical analysis of the exhaust related engine noise. In particular, the analysis of the exhaust related noise would be considered in view of the exhaust gas flow rates and the specified allowable back pressure to design a muffler that would meet specified noise levels. However, most automobile manufacturers produce families of similar vehicles, with each member of the family having either a slightly different version of a common engine, or a different array of engine accessories. Thus, vehicles within such a family would have different exhaust characteristics and/or different performance requirements, and thus, exhaust related noise patterns could vary from one vehicle in a family to another. In most such families of related vehicles, the external envelope required by the various mufflers typically would be constant. However, the internal components of the muffler could vary significantly depending upon the above-described parameters.
The typical prior art muffler with separate tubes, baffles and a tubular outer shell can be redesigned readily to accommodate specific engine or operating characteristics. For example, the manufacturer of the above described prior art muffler would merely have to select tubes having lengths and cross sectional dimensions necessary to meet the specified performance of the particular engine. Similarly, the area encompassed by perforations, louvers or the like in selected tubes of the prior art muffler could readily be achieved using manufacturing equipment and stock materials that are available to the manufacturer of the prior art muffler. Furthermore, the volume encompassed by different chambers within the above described prior art muffler can readily be altered by merely changing the longitudinal position of one or more baffles relative to the tubes of the muffler. Thus variations from one such prior art muffler to the next within a particular family of mufflers could readily be achieved with available stock materials and manufacturing equipment.
The above described prior art muffler has provided adequate acoustical performance and provides for simple variations to muffler configurations that match the performance needs of similar but different engines. However, the above described prior art muffler has several substantial disadvantages. In particular, this typical prior art muffler inherently requires a labor intensive manufacturing process. The large number of separate components also tends to yield a relatively heavy muffler with a corresponding performance penalty for the entire vehicle. Prior art mufflers of this type also are limited to a generally rectangular plan view configuration with essentially fixed locations for the inlet and outlet pipes. These limitations often make it difficult to fit the muffler, the exhaust pipe and the tail pipe into the very limited available space on the underside of the vehicle.
The prior art further includes mufflers manufactured at least in part from stamp formed components. For example, U.S. Pat. No. 4,396,090, which issued to Wolfhugel on Aug. 2, 1983 shows a muffler having a pair of internal plates stamp formed to define pairs of opposed channels. The internal plates are assembled to one another such that each pair of opposed channels defines a tube therebetween. The internal plate defining the channels is disposed within a conventional wrapped outer shell, as in the above described typical muffler.
The prior art also include mufflers consisting only of two opposed shells which are stamped to define a convoluted array of tubes and chambers through which the exhaust gas may travel. Mufflers of this type are shown in U.S. Pat. No. 3,176,791 which issued to Betts et al. on Apr. 6, 1965 and in U.S. Pat. No. 3,638,756 which issued to Thiele on Feb. 1, 1972.
Prior art mufflers having more than two stamp formed components also are known. These prior art mufflers have comprised a pair of internal plates stamped to define opposed channels, with the aligned channels defining tubes therebetween. Selected portions of the tubes have been formed with perforations to permit the expansion of exhaust gases from the tubes. A pair of stamped external shells have been disposed about the internal plates to define a chamber surrounding the perforated formed tubes. The chambers have effectively functioned as expansion chambers to attenuate a broad range of exhaust related noise. Examples of these types of mufflers are shown in British Pat. No. 632,013 which issued to White in 1949; British Pat. No. 1,012,463 which issued to Woolgar on Dec. 8, 1965; Japanese Published Patent Application No. 59-43456 which was published in 1984; U.S. Pat. No. 4,132,286, which issued to Hasui et al. on Jan. 2, 1979; and, U.S. Pat. No. 4,415,059, which issued to Hayashi on Nov. 15, 1983.
Stamp formed mufflers offer the potential of overcoming many of the deficiencies of the above described conventional mufflers with separate tubes and baffles supported in a tubular outer shell and wrapper. In particular, stamp formed mufflers can be formed from many fewer components in manufacturing processes that are well suited to automation. Furthermore, stamp formed mufflers can result in substantially lighter exhaust systems, with corresponding benefits to the vehicular performance.
The above described prior art stamp formed mufflers have not received significant commercial success in the United States. The lack of substantial commercial success has partly been attributable to the poor acoustical performance of these prior art stamp formed mufflers as compared to the acoustical performance of conventional mufflers. In particular, the above described prior art stamp formed mufflers have generally relied upon a single expansion chamber to attenuate most noise. The low frequency noise that may not be adequately attenuated by an expansion chamber has generally remained with the above described prior art stamp formed mufflers. These prior art stamp formed mufflers have received some commercial success in Europe where somewhat higher noise levels have been tolerated. In view of the comparative lack of acoustical tuning, these prior art stamp formed mufflers have not attempted to match the internal construction of each muffler to the particular engine configuration on the vehicle. Thus, a single expansion chamber might be employed for a fairly broad range of engine types.
Recently several significant improvements have been made to stamp formed mufflers. In particular, U.S. Pat. No. 4,700,806 which issued to Jon Harwood on Oct. 20, 1987 shows a muffler formed from stamp formed components and providing the combination of at least one tuning tube and at least one low frequency resonating chamber. Mufflers manufactured in accordance with U.S. Pat. No. 4,700,806 are extremely successful in attenuating both high frequency and low frequency noise and provide acoustical performances equal to or better than conventional mufflers formed from separate tubes, baffles and a tubular outer shell. In view of this superior performance and the other advantages of stamp forming, the mufflers manufactured in accordance with U.S. Pat. No. 4,700,806 have achieved very substantial commercial success in a short period of time. Other improvements relating to stamped mufflers are shown in U.S. Pat. No. 4,736,817 which issued to Jon Harwood on Apr. 12, 1988; U.S. Pat. No. 4,759,423 which issued to Jon Harwood et al. on July 26, 1988; U.S. Pat. No. 4,760,894 which issued to Jon Harwood et al. on Aug. 2, 1988; and, U.S. Pat. No. 4,765,437 which issued to Jon Harwood et al. on Aug. 23, 1988. All of the above described Harwood patents are assigned to the assignee of the subject invention, and the disclosures thereof are incorporated herein by reference.
Although the improvements described in the above identified Harwood patents provide for exceptional acoustical performance, it is desirable to match the acoustical performance of the muffler with each particular engine. For example, each variation of a family of similar engines may require slightly different acoustical tuning. As explained above, optimum acoustical performance is obtained with conventional mufflers by altering the length or cross sectional area of certain tubes, by increasing the total area encompassed by perforations, louvers or apertures, or by moving baffles longitudinally relative to the tubes. Changes of this type can readily be accomplished within the labor intensive manufacturing process of conventional mufflers. Stamp formed mufflers, on the other hand, are formed with carefully manufactured stamping dies having a specified shape. Thus, despite the above referenced manufacturing efficiencies available with stamp formed mufflers, the prior art stamp formed muffler technology is not well suited to minor changes to enable the muffler to match the performance characteristics of various engines. This had not been a particular problem on the earlier versions of European stamped mufflers, because these prior art mufflers did not approach the noise attenuation available with conventional mufflers. The exhaust related noises that would result from altering the characteristics of an engine were well within the broad range of noise levels accepted with vehicles having these prior art stamped mufflers.
Stamp formed mufflers can only be incorporated into the mainstream of original equipment American mufflers by achieving the acoustical performance of conventional mufflers. The need to make separate stamping dies for each engine variation, however, would impose a substantial cost penalty on the stamped muffler.
In view of the above, it is an object of the subject invention to provide stamp formed mufflers that can readily accommodate the acoustical requirements of a plurality of different engines.
It is another object of the subject invention to provide stamp formed mufflers that can achieve different back pressure levels in accordance with the specifications for each of a plurality of different engines.
It is an additional object of the subject invention to provide stamp formed mufflers that can achieve different ranges of low frequency tuning in accordance with each of several different engine requirements.
A further object of the subject invention is to provide stamp formed mufflers that can reduce the number of stamping dies required for manufacturing a plurality of different mufflers.
Still another object of the subject invention is to provide a method for manufacturing a plurality of different stamp formed mufflers